REPORT ON THE TUNICATA. 23 



Aseidians.^ Huxley " and Kowalevsky'' have both described in an admirable manner 

 the process by which buds are developed upon the stolon of Pyrosoma. 



The reproductive organs are formed in diverticula of the posterior median part of 

 the peribranchial cavity, the testis on the left side, the ova on the right. The testis 

 is a lobed organ, which was regarded as the liver until Huxley showed its true 

 nature in 1851. It remains in a rudimentary condition until after the ova have 

 matured. Thus the Ascidiozooids of Pyrosoma, like so many of the Compound 

 Ascidians, are protogynous, and self-fertilisation is prevented. 



The ova are produced one at a time. After fertilisation the development takes place 

 in a diverticulum of the peribranchial cavity, called the ovisac by Huxley, and com- 

 parable with the incubatory pouch found in the genus Colella amongst Compound 

 Ascidians.* 



The segmentation is meroblastic, and an elongated blastoderm is formed on the 

 surface of a mass of yolk. This becomes converted into an embryo, with a tubular 

 alimentary canal, a dorsally placed neural tube, and a pair of laterally placed atrial 

 cavities. This embryo then divides into an anterior and a posterior part. The 

 anterior segments into four pieces, which afterwards develop into the first Ascidio- 

 zooids of the colony, while the posterior part remains in a rudimentary condition, 

 and was called by Huxley the " Cyathozooid ; " it eventually atrophies. As the four 

 Ascidiozooids increase in size, they grow round the Cyathozooid and soon encircle it. 

 The Cyathozooid absorbs the nourishing yolk upon which it lies, and distributes it 

 to the Ascidiozooids by means of a heart and a system of vessels which have formed. 

 When, finally, the Cyathozooid atrophies and is absorbed, its original atrial aperture 

 remains and deepens, to become the central cavity of the young colony which now 

 consists of four Ascidiozooids placed in a ring around where the Cyathozooid was, and 

 all enveloped in a common test. 



The colony gradually increases by the formation of buds from these four original 

 Ascidiozooids.^ Although in most of the species the Ascidiozooids are placed with their 

 ventral surfaces towards the closed end of the colony, and gemmation takes place only 

 from the ventral surface of the Ascidiozooids, still the younger members of the colony 

 are not found, as would be expected, mainly at the closed end of the colony. In most 

 specimens besides those scattered irregularly through the colony, a number of young 

 Ascidiozooids are found round the edge of the common cloacal aperture, while the 

 closed end of the colony, on the other hand, is occupied wholly by adult and, in 

 fact, old-looking Ascidiozooids, not very closely placed. Possibly, as Huxley has 

 suggested, this state of afi"airs may be brought about by a migration of the young 



1 See further on p. 24. - Trans. Linn. Soc. Low]. 1862, p. 211. 



= Archil- f. Ma-r. Anat., Bd. xi. * This Report, Part II. p. 72. 



' For further details as to the embryology of Pijro.'urina, see Huxley, Trans. Linn. Soc. 1862, and Kowalevsky, Uebcr 

 die Entvrickelungsgeschichte der Pyrosoma, Archivf. Mikr. Aiiat.y Bd. xi. 1875, p. 597. 



